(1) Field of the Invention
The present invention lies in the technical field of power plants for single-engined rotary wing aircraft. The invention relates to a method of assisting a pilot of a single-engined rotary wing aircraft during a stage of flight in autorotation. The invention also provides a single-engined rotary wing aircraft including a device for assisting a pilot of the aircraft during a stage of flight in autorotation.
(2) Description of Related Art
A rotary wing aircraft is conventionally provided with at least one main rotor for providing it with lift and possibly also with propulsion, and is generally also provided with a tail rotor, in particular for opposing the yaw torque that is exerted by the main rotor on the fuselage of the aircraft, and that is also used to control yaw movements of the aircraft.
In order to drive rotation of the main rotor and of the tail rotor, the aircraft has a power plant that may include one or more engines.
A distinction is drawn in particular between aircraft of the “single-engined” type in which the power plant has only one engine for driving the main rotor and the tail rotor, and aircraft of the “two-engined” type where the power plant has two engines for this purpose.
It should be observed that throughout this specification, the term “engine” is used to designate a fuel-burning engine such as a turboshaft engine or a piston engine of the kind suitable for use in such a power plant. The term “engine” should be contrasted with the term “motor” where the motor may be driven by electrical power, pneumatic power, etc.
Single-engined aircraft have non-negligible advantages compared with aircraft provided with at least two engines. By way of example, mention may be made of reasonable costs, reduced maintenance operations, and relatively smaller fuel consumption.
Nevertheless, such single-engined aircraft also present drawbacks.
In the event of the single engine being damaged, the power plant, and consequently the single-engined aircraft, presents performance that is degraded, and in the event of the engine failing that can amount to the inability to drive the main rotor and the tail rotor, which leads to a situation that is dangerous, significantly increasing the workload on the pilot of the aircraft. During such a failure, since the main rotor is no longer driven by the power plant, the pilot must begin by entering into a stage of flight in autorotation, and must then perform an emergency landing with the main rotor in autorotation.
A stage of flight in autorotation corresponds to a particular stage of flight in which the aircraft flies, descending without any driving power, but at the cost of a sink rate that is rather large. The term “sink rate” is used to designate the amount of height the aircraft loses per unit time, with this loss of height generally being expressed in feet per minute (ft/min). For example, the descent rate of a single-engined aircraft in autorotation is about 1500 ft/min.
Under such circumstances, the main rotor is caused to rotate by the stream of air passing through it, without making use of a source of energy, and thereby allowing the aircraft to remain maneuverable. When the main rotor is driven in rotation by the relative wind, it remains the seat of stabilized lift that, although less than the weight of the aircraft, nevertheless remains sufficient to brake the descent of the aircraft and to retain control over the aircraft until it has landed.
However, the pilot needs to apply a special piloting procedure and needs to be very attentive firstly to begin by entering into a stage of autorotation when the failure appears, and secondly to continue by performing this always-difficult maneuver all the way to landing. Furthermore, in order to maneuver the aircraft in complete safety during this stage of flight in autorotation and until an emergency landing has been made in an appropriate area, the workload on the pilot is increased. This particular procedure requires great precision and special and frequent training on the part of the pilot of the aircraft. It is a difficult part of piloting aircraft, in particular single-engined aircraft, and is one of the main reasons for which the flight envelope and the use of an aircraft of this type are reduced.
The flight envelope and the missions that are authorized for single-engined aircraft are reduced by the certification authorities that authorize flight. For example, it is forbidden to overfly a large built-up area with a single-engined aircraft. Likewise, the capacities authorized for single-engined aircraft, such as maximum onboard weight, may be limited compared with their real capacities.
One possible solution for improving the performance of single-engined aircraft in this context is the use of a so-called “hybrid” power plant.
In a manner similar to land vehicles, a “hybrid” power plant comprises at least one engine together with at least one electric motor, the mechanical power from the hybrid power plant being delivered either by the engine on its own, or by the electric motor on its own, or by both of them together. For the particular circumstance of single-engined aircraft, a hybrid power plant has only one engine together with at least one electric motor.
By way of example, document FR 2 952 907 describes a hybrid power plant used on a single-engined aircraft having only one engine together with a first electric motor mechanically connected to the main rotor of the aircraft and a second electric motor mechanically connected to its tail rotor. That hybrid power plant also has a set of batteries for the purpose of storing the electrical energy needed for electrically powering the two electric motors.
Those electric motors may be used together with or as a replacement for the engine in order to drive the main and tail rotors. Furthermore, the electric motors are capable of operating in generator mode so as to transform mechanical power into electrical power and thus act as brakes for slowing down the rotors or indeed the engine.
Also known is document FR 2 962 404, which describes the electrical architecture of a hybrid power plant of a rotary wing aircraft. That power plant has at least one engine and at least one electric motor together with a main electricity network and an auxiliary electricity network. The main electricity network serves to provide the aircraft with its general electrical power supply, while the auxiliary electrical network is dedicated to the hybridizing system of the hybrid power plant.
Document EP 2 148 066 describes a hybrid power plant and a method of controlling such a power plant. That power plant has at least one turboshaft engine and at least one electric motor capable of acting together to drive a single main gearbox (MGB). The power delivered by the electric motor is added to the power from each engine.
Document US 2009/0145998 describes a rotary wing aircraft having a first driving power source constituted by a gas turbine and a second driving power source constituted by one or more electric batteries powering an electric motor. Those two power sources are capable of acting simultaneously or independently to drive one or more rotors.
Finally, document WO 2010/123601 describes a vertical takeoff and landing aircraft in which the rotor(s) is/are driven solely by a plurality of electric motors. That aircraft may include a plurality of electrical energy storage means, such as batteries or fuel cells.
In contrast, one of the major drawbacks of using electric motors is storing the electrical energy needed for them to operate. Several solutions exist for storing such electrical energy, e.g. batteries, thermal batteries, or supercapacitors, but each of those solutions has its own specific constraints.
For example, batteries are heavy or indeed very heavy if they are to store a large quantity of electrical energy, whereas supercapacitors are capable of delivering a high level of electrical power, but only for a very limited length of time. Furthermore, thermal batteries are for single use only and have a duration of operation that is limited once they have been activated.
Whatever the means used for storing electrical energy, the quantity of electrical energy that is available remains limited, even though the weight of the electrical energy storage means may be considerable.
Thus, the improvement in performance that it might be possible to achieve by using one or more electric motors within a power plant of an aircraft encounters several limitations associated with storing electrical energy. For example, it is necessary to find a balance between the improvement in the performance of the hybrid power plant and the increase in weight that results from using such means for storing the electrical energy needed to operate the electric motor(s).